System and method for radio signal reconstruction using signal processor

ABSTRACT

A waveform reconstruction circuit receives an rf signal from an antenna, digitizes it, and then generates an undistorted reconstructed waveform. The reconstructed waveform can then be conventionally mixed and demodulated to extract useful signal information with enhanced receiver fidelity and sensitivity.

This application is a continuation of allowed U.S. patent applicationSer. No. 09/178,229, filed Oct. 23, 1998, which in turn is acontinuation of U.S. patent application Ser. No. 08/596,551, filed Feb.5, 1996, now U.S. Pat. No. 5,864,754, both of which are incorporatedherein by reference and priority from both of which is hereby claimed.

FIELD OF THE INVENTION

The present invention relates generally to radio signal processing, andmore particularly to systems and methods for reducing distortion in rfsignals and thus enhancing the fidelity and sensitivity of radioreceivers.

BACKGROUND

Conventional radio receivers function by receiving an rf signal andpreamplifying it, and then processing the signal using a superheterodynestructure. The superheterodyne structure, in its simplest configuration,includes a mixer oscillator which mixes the received signal down to anintermediate frequency (IF) signal. The IF signal is then sent through abandpass filter and demodulated by an envelope detector to recover theinformation (colloquially referred to as “baseband”) that is carried bythe received rf signal.

Of importance to the present invention is the fact that rf signals arecorrupted by environmental factors during transmission. Conventionalsuperheterodyne structures attempt to correct for signal corruption bysuppressing corruption-induced noise using filtering techniques.Unfortunately, such conventional filtering, whether using analog ordigital techniques, suppresses both noise and useful signal, therebyreducing the fidelity of the receiver. In other words, althoughfiltering improves the ratio between useful signal and noise (referredto as the signal-to-noise ratio, SNR), it typically reduces systemfidelity and signal strength.

Further, during demodulation, the envelope detector of a conventionalsuperheterodyne structure effectively demodulates only one-half cycle,for example, the positive half cycle, of the IF signal. Only one half ofthe signal need be used, since the information attached to the positivehalf cycle during transmission is identical to the information attachedto the negative half cycle during transmission. Accordingly, thenegative half of each cycle of the received rf signal is discarded bythe envelope detector, and replaced with a mirror image of the positivehalf.

It happens, however, that either one of the positive or negative half ofa cycle can be distorted asymmetrically from the other half.Consequently, in instances wherein the negative half of a cycle isrelatively uncorrupted, but the positive half cycle is corrupted, theopportunity to use the “best” half of a cycle is lost. Thus, the portionof a corrupted IF signal that is ultimately demodulated and output bythe envelope detector statistically can be expected to be the corrupthalf 50% of the time.

In light of the above discussion as recognized by the present invention,it would be advantageous to analyze both the positive and negativehalves of an rf signal cycle and determine which half is the “best”half, and then extract the useful signal from this “best” half. Asfurther recognized by the present invention, it would be advantageous toaccomplish such analysis prior to the non-linear transformation of therf signal to the IF signal during mixing by the oscillator. Stateddifferently, it would be advantageous to accomplish such analysis priorto mixing, since the mixing function causes certain data in the signalto be irrecoverable and therefore precludes identification of somedistortion and corruption in the “true” signal post-mixing. As stillfurther recognized by the present invention, it would be advantageous toadjust signal gain and tuning “on the fly” to account for transmitterfrequency drift and for sometimes constantly changing received signalstrength at the antenna.

Accordingly, it is an object of the present invention to provide asystem and method for reconstructing a radio signal prior to mixing anddemodulating the signal. Another object of the present invention is toprovide a system and method for reconstructing a radio signal to improvethe extraction of useful portions of the originally transmitted signalthat had been corrupted. Yet another object of the present invention isto provide a system and method for reconstructing a radio signal whichadjusts signal gain and tuning from the antenna on the fly. Stillanother object of the present invention is to provide a system andmethod for reconstructing a radio signal which is easy to use andcost-effective.

SUMMARY OF THE INVENTION

An electromagnetic waveform reconstruction device includes an analog todigital converter (ADC) that is electrically connectable to an antennafor receiving an analog electromagnetic signal therefrom and digitizingthe signal. The ADC outputs the digitized electromagnetic signal to adigital signal processor (DSP), which in turn outputs a reconstructedelectromagnetic signal in accordance with a predetermined reconstructionparadigm. As more fully discussed herein, the DSP is electricallyassociable with a mixer circuit for sending the reconstructedelectromagnetic signal thereto for mixing and demodulating the signal.

Preferably, the electromagnetic signal is an rf signal, and the devicefurther includes a digital to analog converter (DAC) for converting thereconstructed rf signal to an analog reconstructed rf signal, prior tosending the reconstructed rf signal to the mixer circuit. Alternatively,the DSP digitally mixes the reconstructed rf signal and outputs anintermediate frequency (IF) signal to a demodulator.

As envisioned by the preferred embodiment, the DSP includesreconstruction means for effecting method steps to implement thepredetermined reconstruction paradigm. In accordance with the presentinvention, the method steps include receiving both a positive half and anegative half of the digitized rf signal, and then analyzing thepositive and negative halves to identify distorted portions andundistorted portions thereof. At least some of the distorted portionsare removed and replaced with respective replacement portions. Thereby,the reconstructed rf signal is produced, with each replacement portionbeing based on at least some of the undistorted portions.

In one presently preferred embodiment, a controller is electricallyconnected to the DSP. Also, a preamplifier filter circuit (PFC) iselectrically connectable to the antenna and to the ADC for amplifyingand filtering the analog rf signal from the antenna prior to sending theanalog rf signal to the ADC. Moreover, the PFC is also electricallyconnected to the controller. Advantageously, the PFC includes afrequency bandpass filter for attenuating signals having a frequency notequal to a pass frequency, and the controller dynamically establishesthe pass frequency.

Furthermore, in the presently preferred embodiment, the PFC includes anamplifier for increasing, by a gain factor, the amplitude of signalshaving the pass frequency. As intended by the preferred embodiment, thecontroller establishes the gain factor. To this end, the DSP outputs again adjust signal to the controller when the rf signal input to the DSPis characterized by an amplitude outside. of a predetermined amplituderange. Stated somewhat differently, the DSP generates the gain adjustsignal when its input signal is characterized by distortions due to aweak or clipped signal, and the DSP generates the gain adjust signal bydetermining information content of the signal. In response to the gainadjust signal, and the controller dynamically establishes the gainfactor based on the gain adjust signal. If desired, the device of thepresent invention can be combined with an electromagnetic signaltransmitter.

In another aspect of the present invention, an rf receiver includes anantenna and a signal reconstruction circuit electrically connected tothe antenna. Accordingly, the signal reconstruction circuit receives ananalog rf signal from the antenna. Per the principles of the presentinvention, the signal reconstruction circuit generates a substantiallyundistorted reconstructed waveform. A mixer circuit is electricallyassociated with the signal reconstruction circuit for generating anintermediate frequency (IF) signal based on the reconstructed waveform,and a demodulator decodes useful information from the IF signal.

In yet another aspect, a computer-implemented method is disclosed forprocessing a transmitted electromagnetic signal to extract usefulinformation from the signal. The present method includes receiving theelectromagnetic signal and reconstructing it in accordance with apredetermined reconstruction paradigm, and then, after reconstruction,mixing and demodulating the electromagnetic signal to extract usefulinformation therefrom.

In still another aspect, a computer program device includes a computerprogram storage device which is readable by a digital processing system.A program means is provided on the program storage device, and theprogram means includes instructions that are executable by the digitalprocessing system for performing method steps for reconstructing an rfsignal prior to mixing and demodulating the rf signal. The method stepsadvantageously include receiving both a positive half and a negativehalf of the rf signal, and analyzing the positive and negative halves toidentify distorted portions and undistorted portions thereof. The methodsteps further include removing at least some of the distorted portionsand replacing each with a respective replacement portion to therebyproduce a reconstructed rf signal, with each replacement portion beingbased on at least some of the undistorted portions.

In another aspect of the present invention, a device is disclosed fordynamically preamplifying and filtering an rf signal from an antenna,prior to mixing and demodulating the signal to extract usefulinformation from it. The device includes a controller and a preamplifierfilter circuit (PFC) electrically connectable to the antenna and inelectrical communication with the controller for amplifying andfiltering the rf signal. Per the present invention, the PFC includes afrequency bandpass filter for attenuating signals having a frequency notequal to a pass frequency. Additionally, the PFC includes an amplifierfor increasing, by a gain factor, the amplitude of signals having thepass frequency. The controller dynamically establishes/adjusts the passfrequency and gain factor, based on the signal amplitude and distortion.

The details of the present invention, both as to its structure andoperation, can best be understood in reference to the accompanyingdrawings, in which like reference numerals refer to like parts, and inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of the system of the presentinvention;

FIG. 2 is a flow chart showing the overall method steps of the presentinvention; and

FIG. 3 is a flow chart showing the steps of a waveform reconstructionmethod in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring initially to FIG. 1, a system, generally designated 10, isshown for reconstructing an rf waveform signal 12 that has beentransmitted by an rf transmitter 14, before the signal 12 is mixed anddemodulated. As schematically shown in FIG. 1, the rf signal 12 is ananalog, sinusoidally-shaped signal that is relatively smooth andundistorted when transmitted, but which can become degraded anddistorted as it propagates in the direction of the arrow 16 toward an rfantenna 18. Consequently, upon reaching the antenna 18, a negative half20 of the rf signal 12 can have distorted portions 22 and undistortedportions 24. Likewise, a positive half 26 the rf signal 12 can havedistorted portions and undistorted portions as shown. The presentinvention is directed to removing distortions from rf signals, prior tomixing and demodulating the signals incident to the decoding of usefulinformation therefrom, thereby improving the fidelity and sensitivity ofradio receivers.

While the disclosure herein focusses on rf waveform reconstruction, itis to be understood that the principles of the present invention applyequally to other forms of modulated electromagnetic waves that aremodulated as appropriate for the data the waves represent. For example,the principles of the present invention can be applied to processingmodulated light waves that are transmitted through fiber optic bundlesincident to the transfer of computer, video, or voice data.

FIG. 1 shows that the rf signal detected by the antenna 18 is sent to apreamplifying and filtering circuit 28. In accordance with the presentinvention, the preamplifying and filtering circuit 28 includes anamplifying circuit which preamplifies, by a gain factor, the signal fromthe antenna 18. Furthermore, the preamplifying and filtering circuit 28includes a frequency bandpass filter for attenuating signals having afrequency not equal to a pass frequency. As described in greater detailbelow, the pass frequency and gain factor are dynamically establishedunder the principles of the present invention.

Continuing with the description of FIG. 1, an analog to digitalconverter (ADC) 30 is electrically connected to the antenna 18 forreceiving the analog rf signal therefrom. The ADC 30 is structurewell-known in the art that outputs a digitized rf signal in response tothe analog rf input from the antenna 18.

Additionally, a digital signal processor (DSP) 32 is electricallyconnected to the ADC 30. Accordingly, the DSP 32 receives the digitizedsignal from the ADC 30. Per the present invention, the DSP 32 outputs areconstructed rf signal in accordance with a predetermined waveformreconstruction paradigm as more fully disclosed below. The reconstructedwaveform has substantially no distorted portions. Instead, distortedportions in the input signal to the DSP 32 are replaced by smooth,undistorted portions.

As shown in FIG. 1, a digital computer or controller 34 is electricallyconnected to or integrated with the DSP 32. In one preferred embodiment,the DSP 32, controller 34, and ADC 30 establish a waveformreconstruction circuit 35. As intended by the present invention, the DSP32 outputs a gain adjust signal to the controller 34 when the rf signalinput to the DSP 32 is characterized by an amplitude outside of apredetermined amplitude range. In other words, when the amplitude of theinput signal to the DSP 32 is too high or too low, the DSP 32 sends again adjust signal representing this fact to the controller 34.

In turn, the controller 34 is electrically connected to thepreamplifying and filtering circuit 28, and the controller 34dynamically establishes the gain factor of the preamplifying andfiltering circuit 28, based on the gain adjust signal. Moreover, thecontroller 34 can also dynamically establish the pass frequency of thepreamplifying and filtering circuit 28, based on the gain adjust signal,to adjust the signal to optimize reception thereof.

After reconstructing the rf waveform, the DSP 32 sends the reconstructeddigitized signal to a digital-to-analog converter (DAC) 36, whichconverts the digitized output of the DSP 32 to an analog waveform. TheDAC 36 is in turn electrically connected to the mixing circuit of aradio receiver 38. More specifically, the DAC 36 is electricallyconnected to an oscillator mixer 40 of the radio receiver 38, and themixer 40 outputs an intermediate frequency (IF) signal in accordancewith principles well-known in the art, based upon the analog signal fromthe DAC 36. The IF output from the mixer 40 is then sent to ademodulator 42, which decodes the signal to extract useful informationtherefrom. As but one example of how such useful information is used, anaudio speaker 44 can be electrically connected to the demodulator 42 forproducing audio signals, based on the output signal of the demodulator42.

As the skilled artisan will recognize, the configuration shown in FIG. 1is conducive to operably associating the waveform reconstruction circuitof the present invention with existing conventional radio receivers. Inother words, the waveform reconstruction circuit 35 can be implementedin, e.g., a computer chip, and the chip then electrically engaged with aconventional radio receiver between the receiver and antenna asdescribed, for enhancing the fidelity and sensitivity of the radioreceiver. Alternatively, a mixer circuit 32 a can be incorporated in theDSP 32 to digitally implement the function of the mixer 40 afterreconstruction of the waveform. In such an embodiment, the digitizedoutput of the DSP 32 accordingly represents a reconstructed IF signal tobe analogized by a DAC and then decoded by a demodulator.

As can be further appreciated in reference to FIG. 1, the radio receiver38 typically includes one or more tuning control elements, such as, forexample, a knob-like tuning element 46. As is well known in the art, thetuning element 46 is manipulable by a person to establish a channelfrequency selection. As shown in FIG. 1, the tuning element 46 iselectrically connected to the controller 34, such that the controller 34can establish the pass frequency based on the channel frequency. Asstated above, however, once the channel frequency has been set by aperson, the controller 34 can further dynamically vary the passfrequency from the channel frequency as may be required by the gainadjust signal from the DSP 32, to compensate for transmitter 14frequency drift. Stated differently, because the gain adjust signalgenerated by the DSP 32 is based on the received rf signal, thecontroller 34 can dynamically establish the pass frequency and/or gainfactor, based on the received rf signal.

Now referring to FIGS. 2 and 3, the operational steps of the presentinvention can be appreciated. It is to be understood that FIGS. 2 and 3represent logic flow charts of the present reconstruction means forimplementing the predetermined reconstruction paradigm of the presentinvention. As recognized herein, the advantages of the present inventioncan be realized by removing at least some of the distorted portions of areceived waveform and replacing each distorted portion with a respectivereplacement portion that is based on at least some of the undistortedportions of the received waveform. Thereby, a reconstructed rf signal isproduced.

FIGS. 2 and 3 illustrate the logical structure of the waveformreconstruction of the present invention. This logical structure can beembodied in hardware, firmware, or computer program software. When thewaveform reconstruction logic is embodied in software, it will beappreciated that the Figures illustrate the structures of computerprogram code elements that function according to this invention.Manifestly, the software-implemented invention is practiced in itsessential embodiment by a machine component that renders the computerprogram code elements in a form that instructs a digital processingapparatus (that is, a computer) to perform a sequence of function stepscorresponding to those shown in the Figures.

These software instructions may reside on a program storage deviceincluding a data storage medium, such as may be included in the DSP 32.The machine component in such an embodiment is a combination of programcode elements in computer readable form that are embodied in acomputer-usable data medium on the DSP 32. Alternatively, such media canalso be found in semiconductor devices, on magnetic tape, on optical andmagnetic disks, on a DASD array, on magnetic tape, on a conventionalhard disk drive, on electronic read-only memory or on electronic ransomaccess memory, or other appropriate data storage device. In anillustrative embodiment of the invention, the computer-executableinstructions may be lines of compiled C⁺⁺ language code.

Referring particularly to FIG. 2, the waveform reconstruction logic ofthe DSP 32 begins at start oval 48, wherein positive and negative halfcycles of a digitized waveform having distorted and undistorted portionsare received from the ADC 30. At block 50, the gain factor and passfrequency of the preamplifying and filtering circuit 28 are established.Initially, the gain factor is established at a default value, and thepass frequency is established to be equal to the channel frequencyestablished by the tuning element 46 (FIG. 1).

Next, at block 52, the digitized rf signal input to the DSP 32 is read.The present logic proceeds to decision diamond 54 to determine whetherthe amplitude of the input signal is below a predetermined threshold. Ifnot, the logic moves to block 56 to reconstruct the signal as discussedin greater detail below.

From block 56, the logic proceeds to decision diamond 58, wherein it isdetermined whether the amplification of the reconstructed signal exceedsa predetermined value. If not, the logic proceeds to block 60, whereinthe reconstructed signal is mixed (after being analogized, ifappropriate) to generate an IF signal.

On the other hand, if, at decision diamond 58, it is determined that theamplification of the reconstructed signal indeed exceeds a predeterminedvalue, the logic proceeds to block 62, wherein the DSP 32 outputs a gainadjust signal to the controller 34 to cause the controller 34 todecrease the gain factor of the preamplifying and filtering circuit 28.From blocks 60 or 62, the logic moves to block 64, wherein the IF signalis demodulated, and the useful information that is thereby extracted isdisplayed audibly, visually, or indeed stored or otherwise input to adevice requiring the information.

Recall that at decision diamond 54 it is determined whether theamplitude of the input signal is below a predetermined threshold. Inother words, as recognized by the present invention, the signal input tothe DSP 32 should be characterized by an amplitude that is sufficient topermit decoding of useful information from the signal.

If the amplitude is below the threshold, the logic of the presentinvention proceeds to decision diamond 66, wherein it is determinedwhether the pass frequency is equal to the frequency of the received rfsignal. If it is, the logic moves to block 68, wherein the gain adjustsignal from the DSP 32 is generated to cause the controller 34 increasethe gain factor of the preamplifying and filtering circuit 28.

In contrast, if, at decision diamond 66, it is determined that the passfrequency is not equal to the frequency of the received rf signal (i.e.,that the pass frequency is not optimized for receiving the desired rfsignal), the logic moves to block 70. As shown in Figure, at block 70,the controller 34 dynamically varies the pass frequency to set the passfrequency equal to the frequency of the received rf signal. From blocks68 and 70, the logic returns to block 52.

Now referring to FIG. 3, the details of one embodiment of the waveformreconstruction paradigm of the present invention are shown. The paradigmbegins at start oval 72, and moves to block 74, wherein discontinuitiesin the slope (referred to as “dA/dt”) of the input waveform to the DSP32 are identified. As recognized by the present invention, suchdiscontinuities should not exist in a perfect waveform, and consequentlyindicate distorted portions of the waveform. On the other hand, a smoothslope (i.e., dA/dt is a smooth sinusoidal function) indicates anundistorted waveform portion.

From block 74, the logic proceeds to decision diamond 76, wherein it isdetermined whether the corresponding waveform portion in the oppositehalf-cycle that corresponds to the distorted portion is smooth (i.e.,whether dA/dt of the corresponding waveform portion is a smoothsinusoidal function). By “corresponding waveform portion” is meant theportion of the waveform that occupies the segment along the time axis inthe opposite half-cycle from the distorted portion which corresponds tothe segment along the time axis occupied by the distorted portion in itsown half-cycle.

If the test at decision diamond 76 is positive, the logic moves to block78, wherein the distorted portion is replaced with the inverse of thecorresponding waveform portion. Next, the logic moves to decisiondiamond 82, wherein it is determined whether the complete waveform cycle(i.e., one positive half-cycle and its negative half-cycle) has beenanalyzed. If it has been, the process proceeds to block 82, to analyzethe next cycle, returning to block 74. Otherwise, the process proceedsto block 84 to search for the next discontinuity in the current cycle,thence to loop back to decision diamond 76. Also, if the test atdecision diamond 76 is negative, the process skips to decision diamond80.

It is to be understood that the waveform reconstruction paradigm of thepresent invention may use analysis methods other than the one shown inFIG. 3. For example, a fast Fourier transform (FFT) may be used toreconstruct a smooth waveform from a distorted waveform by replacing thedistorted input waveform with a series of smooth regular waveforms froma waveform library, with each replacement waveform having a uniquefrequency and an amplitude based upon its relative contribution to thereconstructed waveform. Accordingly, using FFT analysis, distortedportions of waveforms are replaced by smooth portions, with the smoothportions being based in accordance with FFT principles on theundistorted portions of the input waveform.

As yet another alternative, distorted portions of the input waveform canbe replaced by smooth portions that are based on the undistortedportions of the input waveform using so-called “wavelet analysis”. Inwavelet analysis, small undistorted waveform segments are stored in alibrary and are fitted to the undistorted portions of the input waveformas needed to replace distorted waveform portions. Examples of suchanalysis are disclosed by, e.g., Donoho in “Nonlinear Wavelet Methodsfor Recovery of Signals, Densities, and Spectra from Indirect and NoisyData”, Proceedings of Symposia in Applied Mathematics, Vol. 00. 1993(American Mathematical Society); Basseville et al., “Modeling andEstimation of Multiresolution Stochastic Processes”, IEEE Transactionson Informational Theory, vol. 38. no. 2, 1992 (IEEE); and Coifman etal., “Wavelet Analysis and Signal Processing”, Yale University, 1992,all of which publications are incorporated herein by reference.

While the particular SYSTEM AND METHOD FOR RADIO SIGNAL RECONSTRUCTIONUSING SIGNAL PROCESSOR as herein shown and described in detail is fullycapable of attaining the above-described objects of the invention, it isto be understood that it is the presently preferred embodiment of thepresent invention and is thus representative of the subject matter whichis broadly contemplated by the present invention, that the scope of thepresent invention fully encompasses other embodiments which may becomeobvious to those skilled in the art, and that the scope of the presentinvention is accordingly to be limited by nothing other than theappended claims.

What is claimed is:
 1. A receiver, comprising: a filtering circuitreceiving and filtering an electromagnetic signal; an analog to digitalconverter (ADC) electrically connected to the filtering circuit andoutputting a digitized signal representative of the electromagneticsignal; and a digital processor electrically connected to the ADC, thedigital processor reconstructing the digitized signal, wherein theprocessor includes a mixer generating an intermediate frequency (IF)signal based on the digitized signal, and a demodulator decoding usefulinformation from the IF signal.
 2. The receiver of claim 1, wherein theADC digitizes the signal prior to any analog demodulation thereof. 3.The receiver of claim 1, wherein the filter is controlled and tuned bythe processor.
 4. The receiver of claim 1, further comprising anamplifier connected to the filter, the amplifier being controlled by theprocessor.
 5. The receiver of claim 4, wherein the filter is controlledand tuned by the processor.
 6. The receiver of claim 1, wherein theprocessor reconstructs the digitized signal such that spurious signalelements and noise are removed therefrom.
 7. The receiver of claim 1,wherein the processor reconstructs the digitized signal such that adesired signal is enhanced and amplified.